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Old 29th January 2009, 12:54 PM   #1
clm811 is offline clm811  United States
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Default Help Improve Parts Express Plate Amp

I have a Parts Express#300-794 subwoofer "Plate" amplifier, rated at 250W/4ohms.
http://my.fit.edu/~kozaitis/Speakers...6DataSheet.pdf

The two main power supply capacitors are "Jun Fu" (??) 6,800/80v units. Would it improve the bass power/control if I were to replace these with larger capacitors(within reason)?

I could use either one pair each of 10,000uf/80v, or 15,000uf/80v units(I have these on-hand), but am not sure how much is "too much"...

I do want to improve performance with my 4 ohm sub, but I don't want to "kill" the bridge rectifier or damage the output transistors.

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Old 29th January 2009, 05:05 PM   #2
clm811 is offline clm811  United States
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Default Picture attached

Here is the amp in question.
6,800uf isn't very much for a class AB designed for bass duty...
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Old 29th January 2009, 06:01 PM   #3
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Check out John Curl's comments(post#9) in this thread:

http://www.diyaudio.com/forums/showt...ht=#post342852
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Old 29th January 2009, 07:12 PM   #4
clm811 is offline clm811  United States
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Default Wow!

20,000uf is a lot more than I dare use, esp. due to limited space.

Additionally, I'm concerned about the bridge rectifier that was used(small black block between the big caps, marked "KBPC804").

I've heard "too much capacitance could kill the rectifier bridge"...

Is this true?

If so, how large can/should I (safely) use?

TIA

-Chas
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Old 29th January 2009, 08:26 PM   #5
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Default Re: Wow!

Quote:
Originally posted by clm811

I've heard "too much capacitance could kill the rectifier bridge"...

Is this true?

-Chas
The problem only occurs at power-on, when the cap(s) are charging up and draw a lot of current. One strategy is to put a thermistor into the line (like a CL60 or similar) to limit the current and let the caps charge up more slowly.

Your bridge rectifier is rated at 8A continuous, 40A peak according to a datasheet I checked.

I'm not sure that you will hear a great deal of difference with the added capacitance, though.

Cheers
John
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Old 29th January 2009, 08:38 PM   #6
clm811 is offline clm811  United States
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Quote:
The problem only occurs at power-on, when the cap(s) are charging up and draw a lot of current...Your bridge rectifier is rated at 8A continuous, 40A peak...
So do you experts here think that increasing the cap size to, say, 15,000uf will help or hurt?

Again, this is a subwoofer amp with 3db of boost @30Hz being used to drive a 12" 4 ohm acoustic suspension subwoofer below 100hz in a multi-channel music and home theater system.

-Chas
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Old 29th January 2009, 09:21 PM   #7
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Warning ------ Non expert opinion!!!

I'd look at replacing the bridge rectifier with a biggish 30 amp unit, should only be a couple of dollars, and use as much capacitance as you can fit in the space.

I have the same problem here, except that the power supply is even smaller 42V toroid 320 VA and the caps are 4700uF 60V, but that is what I expected when the whole sub cost less than $100 for a 12 inch driver in a box.
It can't hurt, just may not help as much as a well designed amp in the first place
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Old 29th January 2009, 10:17 PM   #8
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Thanks for the reply, Dog.

I wonder if I can find a 35A bridge with leads to fit the PCB spacing for the 8A unit...

Quote:
... that is what I expected when the whole sub cost less than $100 for a 12 inch driver in a box.
I'm glad YOU got such a deal on your entire subwoofer; Unfortunately, I paid about $150 for just the amp!

My sub's an (older) DIY unit using the venerable NHT1259 woofer(See Pic).

So, I guess I'll go with the 15,000uf caps, then...

-Chas
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Old 29th January 2009, 10:25 PM   #9
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Does it have to fit on the PCB?? can you mount it somewhere else and wire it Point 2 Point??
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Old 30th January 2009, 04:38 AM   #10
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Quote:
Does it[35A bridge] have to fit on the PCB?? can you mount it somewhere else and wire it Point 2 Point??
There isn't a lot of room, since the new caps will be 25% larger in diameter(see closeup of stock parts). I will search for a suitable replacement bridge rectifier.
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